Terahertz (THz) waves refer to electromagnetic radiation in the frequency interval between 0.1 and 10.0 THz (i.e., 1 THz=1012 Hz). THz waves have been investigated for use in various fields, including gas sensing, explosives detection, security screening, biomedical imaging, non-destructive evaluation, and other interdisciplinary studies. However, the use of THz waves is still relatively immature compared to other well-developed sensing and imaging techniques such as the microwave, mid-infrared, infrared, and visible regimes.
As described, some of the most active research areas in THz technology are related to developing THz imaging systems for security screening, biomedical imaging, and non-destructive evaluation for quality monitoring. A compact and real-time THz imaging system is especially needed to compete with other well-developed imaging techniques and to be applied in these areas.
THz imaging systems can be continuous-wave (CW) THz systems or pulsed THz systems. There are several ways to develop a fast CW THz imaging system, including, for example, using a micro-bolometer array detector fast-scanning system with a 2D (two-dimensional) galvanometer and compressed sensing techniques. In compressed sensing, the optical alignment of the system is simple and can operate using only a single point detector. However, there are technical limits to building a real-time THz imaging system based on compressed sensing—namely, a compact and fast THz spatial mask is needed to acquire images. Previous work used several metal masks with different patterns to measure the THz image using compressed image sensing technique, but it is not possible to make a compact and real-time CW THz imaging system using this conventional approach. A compact, efficient, and fast THz spatial mask is therefore necessary to overcome these drawbacks of conventional approaches.
Liquid crystals (LCs) have been used in the visible regime to make state-of-the-art optical devices, such as flat panel displays and screen projectors. LCs have been used as filters, phase shifters, and tunable Bragg reflectors at THz frequencies. LCs typically have large birefringence, comparably small absorption in the THz range, and can be controlled by electric or magnetic fields. In addition, a THz modulator can be implemented using LCs and the THz modulator is the unit cell of THz spatial mask. To design THz optical devices based on LCs, optical properties of LCs in the THz range are required. Several different kinds of liquid crystal mixtures such as, for example, E7 and BL037, have birefringences of 0.1˜0.2 at THz frequencies. Electric or magnetic fields can control the optical properties of LCs and THz optical devices such as a phase shifter, a filter, and a polarizer can be controlled using them. However, THz optical devices controlled by magnetic fields have a limit on the aspect of device integration due to the size of the magnet.
The conventional structure of liquid crystal devices for visible light has optically transparent electrodes to visible light that are opaque at THz frequencies. This makes it difficult to design efficient THz optical devices. Additionally, THz LC devices need approximately wavelength compatible thickness of the liquid crystal layer to control the phase retardation of THz waves. Therefore, the thickness of the liquid crystal layer should be at least several hundredths of micrometer (which is much thicker than for visible light). Also, the operating speed of THz LC devices using conventional designs for visible light is not fast enough for practical applications because the operating speed is inversely-proportional to the thickness of liquid crystal layer.
One prior art approach involves a tuneable THz etalon based on liquid crystals. This etalon uses an ITO (indium tin oxide) layer on the input and output THz windows to control the operating wavelength. However, the ITO layer reflects incident THz waves and is needed to increase the transmission efficiency of THz waves to design practical THz optical devices.
In another prior art approach, a Bragg structure based on LCs can operate in the THz range. The device has electrodes on the side wall to increase the THz transmission and control the operating wavelength, but the response time is the order of hundreds of milliseconds because the liquid crystal layer thickness is so thick. It is not fast enough to design practical devices for real-time imaging systems such as for security cameras or biomedical imaging systems.
Other prior art approaches have developed a phase shifter based on magnetically controlled liquid crystal devices for THz waves, developed a tunable THz wavelength selector based on LCs, and shown that LCs can be used for the polarizing component of THz polarizers. However, these THz optical devices based on LCs are operated by magnetic fields using mechanical control methods, making them unsuitable for use in compact and fast THz systems.
What is needed is an efficient, compact, and fast THz optical device based on liquid crystals for use in THz switches, THz modulators, THz waveplates, and THz spatial masks using liquid crystal that overcomes the drawbacks and limitations of conventional approaches related to transmission efficiency, response time, and size.